1. Field of the Invention
This invention relates to an image processing method and apparatus, wherein frequency emphasis processing, which utilizes an unsharp mask image signal formed from an original image signal, and dynamic range compression processing, which utilizes the unsharp mask image signal, are simultaneously carried out on the original image signal.
2. Description of the Prior Art
The applicant has proposed various image processing methods and apparatuses, wherein frequency emphasis processing or dynamic range compression processing is carried out on an image signal, which represents a radiation image, by using an unsharp mask image signal, and a visible radiation image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, is thereby obtained. (Such techniques are described in, for example, U.S. Pat. Nos. 4,315,318; 4,317,179 and 5,454,044.)
Also, an image processing method and apparatus, wherein both of frequency emphasis processing and dynamic range compression processing are carried out on a single image signal, are disclosed in, for example, Japanese Unexamined Patent Publication No. 6(1994)-292009. In the disclosed image processing method and apparatus, the frequency emphasis processing is carried out in order to compensate for contrast resolution which becomes low due to dynamic range compression, and in order to compensate for a feeling that contrast has become low due to the dynamic range compression. Specifically, in the disclosed image processing method and apparatus, the processing is carried out with Formula (1) shown below. EQU Sproc=Sorg+f.sub.1 (Sus1)+f.sub.2 (Sorg-Sus2) (1)
wherein Sproc represents the processed image signal, Sorg represents the original image signal, each of Sus1 and Sus2 represents the unsharp mask image signal, f.sub.1 represents the monotonously decreasing function, and f.sub.2 represents the monotonously increasing function.
However, with the image processing method disclosed in Japanese Unexamined Patent Publication No. 6(1994)-292009, the problems occur in that, in the region in the vicinity of an edge in the image, at which the image signal value changes sharply, the unsharp mask image signal is affected by the image signal value of the edge in the image. Therefore, an artifact, such as overshoot, undershoot, or an artificial contour, occurs in the image, which is obtained from the processing with the unsharp mask image signal.